Source-drain conductivity

Two of the most important FET configurations make use of a-Si H as semiconductor material for the source - drain conduction channel. One uses a c-Si silicon substrate as gate material (Neudeck and Malhotra, 1975,1976 Matsumura and Nara, 1980 Thompson et al., 1982 Powell et al., 1981 Hayama and Matsumura, 1980 Matsumura et al., 1980 Abdulrida and Allison, 1983), the other uses glass only as a supporting substrate (Le-Comber et al., 1979, 1981 Tuan et al., 1982 Matsumura et al., 1981 Ishibashi and Matsumura, 1982 Lloyd et al., 1983 Nara and Matsumura, 1982 Matsumura and Hayama, 1980), and both employ the a-Si H technology for the semiconductor channel. Merely to give an idea of the differences, schematics of the two types of devices are shown in Fig. 15a,b. The transfer characteristics are reported and discussed in the previously mentioned literature. 

A polymer FET sensor comprises three terminals with the analyte solution directly contacting the thin gate dielectric to form the gate contact, which gates the source-drain conductivity. 

In the case of a phototransistor, the gate contact is transparent to light. This allows photons to pass through the gate and dielectric layer into the semiconductive layer below. The photons can generate hole-electron pairs which split and contribute to current flowing between the source and drain, thereby modulating the source-drain conductivity. 

The operation principle of these TFTs is identical to that of the metal-oxide-semiconductor field-effect transistor (MOSFET) [617,618]. When a positive voltage Vg Is applied to the gate, electrons are accumulated in the a-Si H. At small voltages these electrons will be localized in the deep states of the a-Si H. The conduction and valence bands at the SiN.v-a-Si H interface bend down, and the Fermi level shifts upward. Above a certain threshold voltage Vth a constant proportion of the electrons will be mobile, and the conductivity is increased linearly with Vg - Vih. As a result the transistor switches on. and a current flows from source to drain. The source-drain current /so can be expressed as [619]... 

For correct function of the ISFET, a sufficiently large gate voltage, Vq, must be applied between the leads to the reference electrode and to the substrate, so that a sufficiently large potential difference is formed between the surface and the interior of the substrate for formation of the n-type conductive channel at the insulator/substrate interface. This channel conductively connects drain 1 and source 2, which are connected with the substrate by a p-n transition. On application of voltage Vj between the drain and the source, drain current /p begins to pass. Under certain conditions the drain current is a linear function of the difference between Vq and the Volta potential difference between the substrate and the membrane. 

The panel was produced as follows (Fig. 10.5). A donor film with the structure previously described and coated with a 1-pm DNNSA-PANI/SWNT composite [15] was used to print all conducting circuit elements. First, the large donor film was held by vacuum on the flexible receiver. The gate layer was then printed by selectively exposing the DNNSA-PANI/SWNT donor film as previously described. When exposure was complete the receiver was removed and a l-pm dielectric layer was applied over the whole area. The receiver is then repositioned on to the drum in registry for laser printing of the source/drains and interconnects. The widths of... 

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